It is well known that inadequate protection of active metals such as aluminum, titanium or zirconium may finally result in rapid corrosion which will tend to be local and penetrating if protective surface coatings are cracked in service due to high temperatures.
In semiconductor fabrication technology, as well as in other areas of technology, surface layers are often used to coat the surface of construction parts made of metal in order to protect them against corrosion or to impart desirable properties such as: insulating and dielectric properties, as well as surface hardness.
A particularly convenient type of surface layer for such applications is a layer of oxide of the metal to be protected, produced by the oxidation of the metal surface. The oxidation can be done chemically by immersion of the respective metal in an oxidizing medium, or electrochemically by the method known as anodic oxidation, or anodization. In the method of anodic oxidation the metal to be coated is immersed in a bath of an electrolyte and connected to the positive pole of an external direct current source. The negative pole is connected to an auxiliary electrode immersed in the same bath.
The structure of the oxide film produced on the surface by anodic oxidation depends on the metal, the nature of the electrolyte, its concentration and temperature, and on the voltage applied.
In most applications, for anodic oxidation of aluminum, the electrolyte used is acidic, usually sulfuric acid, but other acids such as chromic acid, phosphoric acid or lactic acid are also often being used. When acidic electrolytes are used for aluminum, the resulting oxide is porous. The pores are known to be perpendicular to the metal surface. Each pore is separated from the metal by a thin compact oxide layer usually called the "barrier layer". The distance between pores, their diameter, and the thickness of the barrier layer are determined by the applied voltage, acid type and concentration, and temperature. Generally, the lower the temperature and concentration, and the higher the voltage, the narrower and less abundant are the resulting pores. The mechanical properties of such oxides are thus enhanced. In order to increase the corrosion resistance of porous anodic oxide films, the pores are often sealed by a subsequent treatment, the simplest one being immersion in boiling water which causes the oxide to increase its volume by hydration.
For special applications using aluminum, such as when barrier type films are required, neutral electrolytes are used. Typical electrolytes are aqueous solutions of compounds such as ammonium citrate, ammonium tartrate, etc. The oxides formed in neutral electrolytes are compact and non-porous. Furthermore, the oxides formed by any type of bath on a number of anodically oxidizable metals other than aluminum are also compact and non-porous.
The use of oxide films produced by the known anodic oxidation techniques for corrosion protection of the metal, is limited to low temperatures. When the oxidized metal is subjected to an elevated temperature, the oxide layer typically cracks by tensile stresses which are due to the difference in the expansion coefficient between the metal and the oxide (e.g., 5.times.10.sup.-6 /.degree. C. for aluminum oxide, and 25.times.10.sup. 6/.degree. C. for aluminum metal). Such cracks create a pathway for the corrosive environment to attack the underlying bare metal, thereby permitting penetrating corrosion to occur which can result in structural damage to the part and loss of adhesion and flaking of the oxide layer. Additionally, any water used to seal porous anodic films is evaporated at such temperatures and the films return to being susceptible to damage by corrosive environments.
The problems of corrosion have been greatly intensified in the last forty years by developments in jet engines, nuclear energy and computer manufacturing. Elevated temperatures are very common in fabrication chambers in the semiconductor and other industries, combined with extremely corrosive environments such as fluorinated gas in Chemical Vapor Deposition (CVD) chambers, for example, or in hot parts of aircraft engines and external parts of aircraft subject to high flying velocities. In certain types of equipment associated with nuclear reactors, not only are metals exposed to corrosive chemicals and elevated temperatures, but the nuclear reactor metals are subjected to hydrogen and deuterium which may induce changes in the physical properties of the metal, such as ductility.
The known anodization processes are therefore incapable of affording protection under such conditions. Frequent failures are thus encountered in critical parts of such equipment, particularly when operating at high temperatures of several hundred degrees centigrade, and rapid loss of metal occurs by corrosion. This, in turn, results in the need for frequent replacement of parts, loss of production time and contamination of electronic microcircuitry with particles of corrosion products. In supersonic aircraft, even melting of the metal may result due to loss of the insulating oxide coating by thermal cracking.
The above brief review of the problem clearly indicates the need for an improved method to obtain adequate protect, ion of a metal by an oxide layer which persists for prolonged periods of time even after use at high temperatures.
U.S. Pat. No. 3,551,303 to Suzuki et al relates to a method for forming anodic oxide film on aluminum or an aluminum alloy for the purpose of electrical insulation. The problem being solved by the Suzuki et al patent is different from the problem being addressed by the present invention. The Suzuki et al patent addresses the problem that the anodic oxide film has little flexibility and cracks on elongation of the surface by only 0.4-5%, such as being subjected to bending. When subjected to such tensile stress, the cracks which are formed reduce the insulating property of the film if their aperture is too wide. There is no problem of the film actually falling off of the aluminum, as the patent indicates that the adhesive property of the film is excellent. The only disadvantage is that the breakdown voltage of the film becomes lower when the conductor is bent with a radius of curvature not larger than about 20 times as large as the diameter or thickness of the conductor. This problem is solved in the invention of the Suzuki et al. patent by first forming the anodic oxide film on the surface of the aluminum or aluminum alloy at a thickness smaller than the thickness of the desired final film. Then cracks are intentionally formed over the region of the anodic oxide film by elongating the film or by subjecting the conductor having the anodic oxide film to a rapid temperature change, and using the difference between the thermal expansion coefficient of aluminum or aluminum alloy and that of the anodic oxide film for formation of the cracks. The specific extent of heat treatment is nowhere disclosed. Following intentional crack formation, anodic oxidation is again carried out so as to increase the thickness of the anodic oxide film above the thickness at the time of the original crack formation. According to the method of Suzuki et al., the previously formed cracks extend to the metal through the thick oxide film during bending in service in the larger numbers and the narrower aperture typical of those formed in thin oxides.
U.S. Pat. No. 4,052,273 to Aronson et al. discloses a method of anodizing porous sintered tantalum material, suitable for making a porous tantalum capacitor pellet or slug having decreased current leakage. After such a pellet is anodized at a maximum predetermined desired voltage, it is removed from the anodizing bath and heated to a temperature of between 150.degree.-300.degree. C. for at least three minutes, and then returned to the anodizing bath and subjected to more electrical current. The heating and reanodizing steps may be repeated. The sole purpose of this heat treatment is to decrease the current leakage of the capacitor anode. U.S. Pat. N. 4,781,802 to Fresia discloses a similar method.
Japanese patent 60/033,393 discloses a method for electrolytically coloring aluminum or aluminum alloy by anodically electrolyzing the aluminum or aluminum alloy in a phosphoric acid solution to form an anodic oxidation layer, electrolyzing in an aqueous electrolyte containing metal salt with an alternating current, heat treating at 300.degree.-400.degree. C., dipping in a phosphoric acid bath to rapidly cool the aluminum or aluminum alloy to room temperature, and then anodically electrolyzing in a phosphoric acid solution. The sole purpose of the method is to provide a unique coloring effect.
U.S. Pat. No. 3,864,220 to Denning et al discloses a method for reducing hydrogen embrittlement of nuclear reactor structural parts made of zirconium or zirconium alloy. The part is first surface anodized and then subjected to heat treatment in an oxidizing atmosphere. There is no subsequent re-anodization step.